Nanowire array and nanowire solar cells and methods for forming the same

ABSTRACT

Homogeneous and dense arrays of nanowires are described. The nanowires can be formed in solution and can have average diameters of 40-300 nm and lengths of 1-3 μm. They can be formed on any suitable substrate. Photovoltaic devices are also described.

CROSS-REFERENCES TO RELATED APPLICATIONS

This patent application is a divisional of U.S. patent application Ser.No. 10/868,421, filed Jun. 14, 2004, now U.S. Pat. No. 7,265,037, issuedon Sep. 4, 2007, which, in turn, claims the benefit of U.S. PatentApplication No. 60/480,256, filed Jun. 20, 2003, both of which areherein incorporated by reference in their entirety.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH OR DEVELOPMENT

The invention described and claimed herein was made in part utilizingfunds supplied by the United States Department of Energy under contractNo. DE-AC03-76SF000-98 between the United States Department of Energyand The Regents of the University of California. The government hascertain rights to the invention.

BACKGROUND OF THE INVENTION

The first solar cells were fabricated in the mid 1950s from crystallinesilicon wafers. At that time, the most efficient devices converted 6% ofsolar power to electricity. Advancements in solar cell technology overthe past 50 years have resulted in the most efficient Si cell being at25% and commercial Si modules being at 10%.

Despite these efficiencies, the high cost of manufacturing conventionalsolar cells limits their widespread use as a source of power generation.The construction of conventional silicon solar cells involves four mainprocesses: the growth of the semiconductor material, separation intowafers, formation of a device and its junctions, and encapsulation. Forcell fabrication alone, thirteen steps are required to make the solarcell and of these thirteen steps, five require high temperatures (300°C.-1000° C.), high vacuum or both. In addition, the growth of thesemiconductor from a melt is at temperatures above 1400° C. under aninert argon atmosphere. To obtain high efficiency devices (>10%),structures involving concentrator systems to focus sunlight onto thedevice, multiple semiconductors and quantum wells to absorb more light,or higher performance semiconductors such as GaAs and InP, are needed.These options all result in increased costs.

Another problem with conventional solar devices is the high cost ofmanufacturing materials. The amount of silicon needed for 1 kW of moduleoutput power is approximately 20 kg. At $20/kg, the material costs forelectronic grade silicon are partially subsidized by the chipmanufacturing sector. Other materials such as GaAs, which aresynthesized with highly toxic gases, are a factor of 20 higher in costat $400/kg. Because solar cells are large area devices, such materialcosts hinder the production of inexpensive cells. As a result, thin filmdevices, which have active layers several microns thick of amorphous Si,CdTe, and CuInSe₂ are being explored. Also, in 1991, O'Regan et al.reported the invention of a novel photochemical solar cell comprised ofinexpensive TiO₂ nanocrystals and organic dye (O'Regan et al. Nature353, 737 (1991)).

Embodiments of the invention improve upon such conventional devices andaddress the above problems individually and collectively.

SUMMARY OF THE INVENTION

Embodiments of the invention are directed to nanowire arrays, devicesthat use the nanowire arrays, and methods for making the same.

One embodiment of the invention is directed to a method comprising:providing a substrate; depositing ZnO nanocrystals on the substrateusing a dip coating process; and contacting the substrate with asolution of zinc nitrate hexahydrate (Zn(NO₃)₂.6H₂O) and methenamine(C₆H₁₂N₄).

Another embodiment of the invention is directed to a method comprising:providing a substrate; depositing semiconductor nanocrystals on thesubstrate using a dip coating process; contacting the substrate with asolution comprising a semiconductor precursor; and forming an array ofnanowires, wherein the nanowires comprise a semiconductor.

Another embodiment of the invention is directed to a method comprising:providing a substrate; depositing semiconductor nanocrystals on thesubstrate; contacting the substrate with a solution comprising asemiconductor precursor and a polyamine; and forming an array ofnanowires, wherein the nanowires comprise a semiconductor.

Another embodiment of the invention is directed to a device comprising:a substrate; and an array of nanowires on the substrate, wherein eachnanowire includes an aspect ratio greater than about 20 or even 120, anda length greater than about 15 or 20 microns.

Another embodiment of the invention is directed to a method for forminga branched network of metal oxide semiconductor wire structures, themethod comprising: providing a substrate; depositing a first pluralityof semiconductor nanocrystals on the substrate; contacting the substratewith a solution comprising a semiconductor precursor; forming an arrayof nanowires on the substrate; depositing a second plurality ofsemiconductor nanocrystals on the array of nanowires; and formingbranches on the array of nanowires using the deposited second pluralityof nanocrystals.

Another embodiment of the invention is directed to a device comprising:a substrate; and a branched network on the substrate, wherein thebranched network comprises an array of nanowires and branches on thenanowires in the array of nanowires, wherein the branched networkcomprises a metal oxide semiconductor.

Another embodiment of the invention is directed to an optoelectronicdevice comprising: a first conductive layer; a second conductive layer;an array of semiconductor nanowires between the first and secondconductive layers; and a charge transport medium between the first andsecond conductive layers.

These and other embodiments of the invention are described in detailbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flowchart illustrating some steps in a method accordingto an embodiment of the invention.

FIG. 2 shows images showing how a nanowire array is created. The firstphotograph shows ZnO quantum dots, while the second photograph showsnanowires grown from the quantum dots. The scale bar equals 1 μm in thebottom photograph.

FIG. 3 shows a transmission electron micrograph of a cluster of ZnOnanowires removed from a 1.5-hour array. The scale bar is 100 nm. Theinset is the [100] electron diffraction pattern of an isolated, singlenanowire from a different region of the sample. The growth axis is alongthe [001] direction.

FIG. 4 shows a graph of I_(SD) v. V_(G) (source drain current vs. gatevoltage).

FIG. 5 shows images of a 4 inch wafer and nanowires at different scales.From the upper left and moving clockwise, the scale bars are 2 μm, 500nm, 200 nm, and 1 μm.

FIG. 6A shows a ZnO nanowire array on a two inch, flexible PDMSsubstrate. The photograph shows a flexed array of nanowires on PDMS.

FIG. 6B shows an SEM image showing the array morphology on a flexiblesubstrate. The scale bar is 1 μm.

FIG. 6C shows a low magnification of an SEM image showing cracks formedin the array after handling. The scale bar is 5 μm.

FIG. 7 shows a graph of nanowire diameter vs. length. The effect ofusing and not using polyethylenimine (PEI) is illustrated.

FIG. 8A shows a nanowire array on a substrate for exemplary use in anorganic-inorganic hybrid photovoltaic device. The nominal aspect ratiois about ten. The scale bar is 200 nanometers.

FIG. 8B shows a nanowire array on a substrate for exemplary use in a DSCor dye sensitized cell. The nominal aspect ratio is greater than about120. The scale bar is 5 microns.

FIG. 9A shows a schematic drawing of a conventional DSC.

FIG. 9B shows schematic drawing of a DSC according to an embodiment ofthe invention.

FIG. 10 shows a graph of current density vs. bias (V).

FIG. 11 shows a graph of cell efficiency vs. roughness factor.

FIG. 12 shows a schematic top view of nanowires in an array.

FIG. 13 shows a schematic cross-sectional view of an organic-inorganichybrid cell according to an embodiment of the invention.

FIG. 14 shows various views of wire composites according to embodimentsof the invention.

FIG. 15 is an x-ray diffraction graph.

FIG. 16 shows the bias/current relationship in accordance an embodimentof the invention.

FIG. 17 shows a schematic cross-sectional view of a solid statesensitized cell with a nanowire array.

FIG. 18 shows a cross-sectional view of a light emitting diode with ananowire array.

FIG. 19 shows a flowchart illustrating a method according to anotherembodiment of the invention.

FIG. 20 shows a schematic illustration of a branched network on asubstrate.

DETAILED DESCRIPTION

Embodiments of the invention are directed to nanowire arrays, devicesincluding nanowire arrays, and methods for making the same. Inembodiments of the invention, synthetic methods can be used to producehomogeneous and dense arrays of nanowires. The nanowires can be grown onvarious substrates under mild aqueous conditions. Solution approaches toproducing nanowires are appealing, because they can use low temperatureprocessing steps that are suitable for scale-up.

In one exemplary embodiment of the invention, homogeneous and densearrays of ZnO nanowires were synthesized on 4-inch silicon wafers usinga mild solution process at 90° C. Uniform ZnO nanocrystals were firstdeposited on the substrates to act as seeds for subsequent hydrothermalnanowire growth on the substrates. This procedure yielded singlecrystalline wurtzite ZnO nanowires grown along the [0001] direction. Thenanowires were oriented perpendicular to the wafer surface. Bycontrolling the reaction time, average diameters of about 40 to about300 nanometers and lengths of about 1 to about 3 μm were obtained forthe nanowires.

Although zinc oxide (ZnO) nanowires are described in detail, it isunderstood that embodiments of the invention are not limited thereto.For example, instead of zinc oxide, other semiconductors such as metaloxide semiconductors (e.g., titanium oxide, zinc oxide, tin oxide, etc.)can be used in the nanowire arrays according to embodiments of theinvention.

The resulting nanowire arrays according to embodiments of the inventionhave unexpectedly high surface areas and unexpectedly good electricalproperties. The inventive methods of fabricating the nanowire arraysensure that a majority of the nanowires in the arrays directly contactthe substrate and provide continuous pathways for carrier transport.This is a desirable feature for electronic devices based on thesematerials. Highly efficient carrier transport (e.g., electron transport)through the substrate, to the nanowires, and though the nanowires isdesirable, since this results in higher energy conversion efficienciesin devices such as photovoltaic devices.

The nanowire arrays according to embodiments of the invention can beused in any suitable device including optoelectronic devices such asphotovoltaic devices (which includes solar cells). The term“photovoltaic device” includes those device architectures known in theart. Exemplary photovoltaic devices are described in, for example,Science, Vol. 295, pp. 2425-2427, Mar. 29, 2002, the contents of whichare incorporated by reference. Such devices include dye sensitized cells(DSCs) or Grätzel cells, solid state solar cells, and organic-inorganichybrid photovoltaic cells. In an organic-inorganic hybrid photovoltaiccell, a semiconducting or conducting polymer is used in conjunction withthe array of nanowires. These and other types of devices are describedin further detail below.

Embodiments of the invention can also be used in other devices,including but not limited to, acoustic wave filters, photonic crystals,light emitting diodes, photodetectors, photodiodes, optical modulatorwaveguides, varistors, and gas sensors.

I. General Methods For Forming Nanowire Arrays

FIG. 1 shows a flowchart illustrating steps in methods according toembodiments of the invention. First, a substrate is provided (step 400).For example, the substrate may comprise glass with indium tin oxide(ITO) coated on it, or may comprise a fluorine doped tin oxide.Nanocrystals are then deposited on the substrate, preferably through aself-assembly process (step 402). The nanocrystals may comprise the samematerial that will be present in the nanowires. For example, thenanocrystals may comprise a semiconducting material such as zinc oxideand the resulting nanowires could also be made of zinc oxide. Afterdepositing the nanocrystals on the substrate, a solution then contactsthe deposited nanocrystals. The solution may be formed from asemiconductor precursor (e.g., a salt) and a nanowire growth material.For example, the solution may comprise a semiconductor precursor such aszinc nitrate and a nanowire growth material such as an amine. In someembodiments, the amine is methenamine. As will be described in furtherdetail below, one can get better aspect ratios with zinc nitrate,methenamine and PEI. After the solution contacts the depositednanocrystals, nanowires are formed (step 404). After growing thenanowires, the nanowire array can be removed from the solution, dried,and then further processed. For example, additional layers may be formedon the nanowire array so that an optoelectronic device is formed. Eachof these process steps and additional processing steps are described infurther detail below.

First, a substrate is provided (step 400). The substrate may compriseany suitable material. For example, the material may be organic and/orinorganic in nature, may comprise a conducting or a semiconductingmaterial, and/or may be transparent or semi-transparent or opaque. Thematerial may also be rigid or flexible. Exemplary substrates includesemiconductors such as silicon and gallium arsenide, metals such astitanium foil, metal oxides such as titanium oxide, tin oxide, zincoxide, and indium tin oxide, polymers such as semiconducting polymers,insulating polymers, etc. The substrate may also be a composite materialwith one or more sublayers. For example, the substrate may comprise aflexible, insulating, polymer base layer which may be coated with aconductive film. Any of these characteristics and/or materials can be inthe substrates according to embodiments of the invention.

After obtaining the substrate, nanocrystals are deposited on thesubstrate, preferably by a self-assembly process (step 402). As usedherein “depositing” includes forming nanoparticles directly on thesubstrate. It also includes preforming the nanoparticles and thenplacing them onto the substrate surface through the use of a gaseous orliquid medium. Pacholski (Pacholski et al., Angew. Chem. Int. Ed.,41:1188 (2002)) describes how one can make ZnO nanocrystals (thecontents of which are incorporated by reference in its entirety for allpurposes). Other suitable deposition processes include spin coating,blade coating, roller coating, spraying, curtain coating, dip coating,inkjet printing and screen printing. In another embodiment, a zinc salt(e.g., zinc acetate) can be added to a substrate and it can be heated toform ZnO on the substrate. More generally, depositing a metal ion ontothe surface of a substrate and heating it in the presence of oxygen canform an appropriate oxide.

In embodiments of the invention, the deposition process is preferably adip coating process. Unlike a spin coating process, when seeding asubstrate with a nanocrystal material such as ZnO, a very thin layer ofnanocrystals can be produced. For example, a very thin layer of ZnOnanoparticles about 10-15 nanometers in thickness (or less) may beformed on the substrate after a dip coating process is performed. Incomparison, a spin coating process may leave a 50-200 nanometer film ofnanocrystals on a substrate. The use of a dip coating process to depositnanocrystals on a substrate allows one to grow the array of nanowiresvery close to the substrate. When the nanowires are formed on the verythin layer of ZnO nanocrystal particles, their ends are in virtuallydirect contact with the substrate. A large intermediate particle layeris not between the array of nanowires and the substrate. This allows forbetter electron transport between the substrate and the nanowires. Whenthe nanowires are used in a device such as a photovoltaic device, thiswill result in improved conversion efficiency.

The nanocrystals on the substrate may have any suitable size (however,the final wire diameter and distribution can be dependent on the size ofthe nanocrystals), and may comprise any suitable material. For example,in some embodiments, the nanocrystals may be about 5 to 10 nanometers indiameter. In other embodiments, the nanocrystals may be smaller orlarger than this. They may contain the same materials that are presentin the nanowires.

In some embodiments, before depositing the nanocrystals on thesubstrate, the substrate may be briefly etched with an acid to providethe surface of the substrate with a positive charge (e.g., caused by thepresence of protons). Suitable acids include HCl, HNO₃, and other acids.In other embodiments, an electrical bias could be provided to thesubstrate surface to charge it. The application of electrical biases tosubstrates is well known to those of ordinary skill in the art.Providing a positive charge to the substrate surface improves theadhesion of the nanocrystals onto the substrate. The nanocrystals may beoppositely charged, and may adhere to the substrate by electrostaticforce.

To further improve the adhesion of the nanocrystals to the substrate,the substrate may be optionally heated after they are deposited. Forexample, a substrate and nanocrystals can be annealed at 150° C. ormore, for a predetermined period of time, to ensure that the nanocrystalparticles adhere to the substrate surface.

After depositing nanocrystals on the substrate, a solution contacts thenanocrystals (step 404). The solution can be formed from a liquidmedium, a semiconductor precursor, and a nanowire growth material.Suitable concentrations of the solution components and mixing procedurescan be determined by those of ordinary skill in the art. Exemplaryconcentrations and mixing procedures are described herein in specificexamples.

In embodiments of the invention, the semiconductor precursor maycomprise a metal salt that can dissociate in solution. The metal of themetal salt may include metals such as Ti, Zn, Sn, etc. Suitable saltsfor forming zinc oxide nanowires may be zinc nitrate, zinc acetate, andhydrated forms thereof.

The nanowire growth material may comprise any suitable material that isadapted to induce the growth of nanowires in solution using thesemiconductor precursor. Examples of nanowire growth materials includeamines, phosphonic acids, and/or carboxylic acids. In some embodiments,the amine may be methenamine (C₆H₁₂N₄), which is a highly water soluble,non-ionic tetradentate cyclic tertiary amine. It is also sometimescalled hexamethylenetetramine (HMTA).

In preferred embodiments, the nanowire growth material may furthercomprise a polyamine such as a polyethylenimine (PEI). As illustrated infurther detail below, surprising and unexpected results are producedwhen PEI is used. PEI is a cationic polyelectrolyte and is believed toselectively hinder lateral growth of the nanowires in solution. UsingPEI in the solution, the aspect ratios of the nanowires increased toabove 120, or even 150. This is compared to aspect ratios of 20-25 orless when a polyamine is not used. Further data regarding the unexpectedresults associated with the use of polyamines are provided below.

The liquid medium may be organic and inorganic in nature. As illustratedbelow, the liquid medium may comprise water. The liquid medium may alsocomprise or be formed from an alcohol such as ethanol.

After the solution is prepared, the solution may contact the substrateand the deposited nanocrystals for an amount of time, and at atemperature and a pressure sufficient to cause the formation ofnanowires on the substrate. For example, the substrate and/or thesolution may be heated between about 60° C. to about 95° C. for about0.5 to about 6 hours at ambient pressure to cause the formation of thenanowire array. The solution could also be agitated during the nanowireformation process using a mixer or a stirrer.

An array of nanowires is then formed in the solution (step 406). As usedherein, a “nanowire array” includes at least some substantially linearnanowires extending substantially perpendicular to the surface of asubstrate. Nanobranches may or may not be on the nanowires in the array.Embodiments with nanobranches are described in further detail below.

The resulting nanowires may comprise a semiconductor material. Examplesof suitable semiconductor materials include zinc oxide (e.g., ZnO),titanium oxide (e.g., TiO2), tin oxide (e.g., SnO), etc. In someembodiments, the nanowires preferably comprise a metal oxidesemiconductor. The nanowires may be single crystalline orpolycrystalline, and may be doped or undoped.

The nanowires in the nanowire array may also have any suitabledimensions. For example, each of the nanowires in an array of nanowiresmay have a diameter of from about 1 nanometer to about 200 nanometers.They may have lengths from several microns or more (e.g., greater thanabout 10 or about 20 microns). The nanowires preferably have high aspectratios. For example, the aspect ratios of the nanowires can be betweenabout 10 to about 500, or more. The widths of the nanowires may be about100 nanometers or less in some embodiments. For example, each nanowiremay have a diameter between about 40 to about 80 nanometers. Somenanowires in the array may not have the above noted dimensions.

In one exemplary embodiment of the invention, well-aligned ZnO nanowirearrays were grown. ZnO nanocrystals about 5 to about 10 nanometers indiameter were dip coated several times onto a 4-inch Si (100) wafer toform a <15 nanometer thick film of crystal seeds. The ZnO nanocrystalswere prepared according to a modified method of Pacholski (Pacholski etal., Angew. Chem. Int. Ed., 41:1188 (2002)), the contents of which areincorporated by reference in its entirety. A 0.03 M NaOH solution inethanol was added slowly to 0.01 M zinc acetate dehydrate in ethanol at60° C. and stirred for two hours. The resulting nanoparticles werespherical and stable for at least one week in solution and longer whenkept at almost freezing conditions.

After uniformly coating the silicon wafer with ZnO nanocrystals,hydrothermal ZnO growth was carried out by suspending the waferupside-down in an open crystallizing dish filled with an aqueoussolution of zinc nitrate hydrate (0.025M) and methenamine (0.025M) at90° C. The reaction time in this example was from about 0.5 to about 6hours. The wafer was then removed from solution, rinsed with deionizedwater and dried. A scanning electron microscope (SEM) was used toexamine the morphology of the nanowire array across the entire wafer,while single nanowires were characterized by transmission electronmicroscopy (TEM). Nanowire crystallinity and growth direction wereanalyzed by X-ray diffraction and electron diffraction techniques.

SEM images taken of several 4-inch samples showed that the entire waferwas coated with a highly uniform and densely packed array of ZnOnanowires. X-ray diffraction data suggest a wurtzite ZnO pattern with anenhanced (002) peak due to the vertical orientation of the nanowires. Atypical 1.5-hour synthesis yielded wires with diameters ranging betweenabout 40 to about 80 nm and lengths of about 1.5 to about 2 μm. With ameasured number density on the order of about 10¹⁰ cm⁻², these arrayshad a ZnO surface area of, conservatively, at least about 50 cm² per cm²of substrate (˜10 m² g⁻¹). The average size of the nanowires increasedwith longer reaction time, up to about 200 to about 300 nm wide by about3 μm long for a 6 hour experiment. High magnification SEM imaging of a 6hour sample revealed that the surfaces of these solution-grown wires arerough compared to the gas-phase arrays, which might be expected based onthe different crystallization environments. Also, a substantialpercentage of the nanowires fuse together after longer reaction times.

FIG. 2 shows images showing how a nanowire array including longnanowires can be created. The first photograph (created using AtomicForce Microscopy or AFM) shows dip coated ZnO quantum dots ornanocrystals on a substrate. The second photograph shows nanowires grownfrom the nanocrystals. In the bottom photograph, the scale bar equals 1μm. The cross-sectional SEM view of the array suggested that the ZnOnanowires grow nearly vertically and penetrated a thin (<15 nm) layer ofnanocrystals. As noted above, it is desirable to make the nanoparticlelayer as thin and continuous as possible (ideally a few particlediameters or less in thickness) for electronic applications.

FIG. 3 shows a TEM characterization of individual nanowires removed fromthe arrays. It indicates that they are single crystalline and grow inthe [0001] direction. The cluster morphology shown in the image iscommon and suggests that multiple nanowires often grow from a singleaggregate of ZnO nanoparticles attached to the substrate.

FIG. 4 shows electrical characteristics for a single ZnO nanowire.Specifically, FIG. 4 shows I-V curves at various gate biases for ananowire with a diameter of 75 nm, showing n-type behaviour and aresistivity of 0.65Ω cm. The left inset shows the correspondingI_(SD)-V_(G) curve at V_(SD)=100 mV. The ON-OFF ratio is 10⁵ at 50volts. The right inset shows an SEM image of a ZnO NW-FET.

FIG. 5 shows a 4 inch wafer and various SEM images of nanowires.

FIGS. 6A-6C show embodiments using flexible substrates. For example,FIG. 6A shows the results of nanowire array growth on a flexible 2-inchpolydimethylsiloxane (PDMS) substrate. This array is similar to thosegrown on silicon, except that a network of microscale cracks form due tothe inflexibility of the ZnO nanowire film. See FIGS. 6B and 6C.

FIG. 7 shows a graph, which correlates nanowire length and diameter atdifferent growth times with and without PEI addition. The longest arrayspresented have nanowires with lengths exceeding ˜20 μm. As shown in FIG.7, when PEI (or other polyamine) is used, a two, three, or fourfoldincrease in the lengths of the nanowires can be achieved, as compared toembodiments that do not use PEI.

FIG. 8A shows a nanowire array for an organic-inorganic hybrid cell (ordevice) where the nanowires have aspect ratios of about 10 (formedwithout a polyamine). The array could also be formed with a polyamine.

FIG. 8B shows a nanowire array for a DSC where the nanowires have aspectratios of about 120 or more (formed with a polyamine).

Once a nanowire array has been formed on a substrate, an optoelectronicdevice such as a photovoltaic device may be formed. Such devices aredescribed in further detail above and below.

II. Optoelectronic Devices Including The Nanowire Arrays

Other embodiments of the invention are directed to an optoelectronicdevices. As used herein, optoelectronic devices include devices that caneither produce light or can convert light to electricity. They mayinclude a first conductive layer, a second conductive layer, an array ofsemiconductor nanowires (with or without nanobranches) between the firstand second conductive layers, and a charge transport medium between thefirst and second conductive layers. Such optoelectronic devices mayinclude DSCs, organic-inorganic hybrid photovoltaic devices, solid-statesensitized solar cells, and light emitting diodes. Details of these andother devices are provided below.

The charge transport medium that is used depends on the type ofoptoelectronic device that is produced. For example, the chargetransport medium in a DSC may comprise an electrolyte. By comparison, anelectrolyte may not be present in an organic-inorganic hybridphotovoltaic device. In embodiments of the invention, the chargetransport medium may fill or impregnate the spaces between the nanowiresto form a wire composite. In some embodiments of the invention, the wirecomposite may include 5-95% by volume of the charge transport mediumand/or 9-95% by volume of the nanowires.

The nanowire arrays in the optoelectronic devices can be formed usingany of the solution processes described above (and below). In otherembodiments, the nanowire arrays may be formed using vapor phaseprocesses. Vapor-phase processes are described in Yang et al. andgenerally require high temperatures. Yang et al., Adv. Func. Mater.,12:323 (2002); Yao et al., Appl. Phys. Lett., 81:757 (2002).Accordingly, the solution based processes described herein arepreferred.

A. DSCs (Dye Sensitized Cells)

The DSC is currently the most efficient and stable excitonic photocell.Central to this device is a thick nanoparticle film that provides alarge surface area (roughness factor ˜1000) for light harvesting.However, nanoparticle DSCs rely on trap-limited diffusion for electrontransport, a slow mechanism that can limit device efficiency, especiallyat longer wavelengths.

In a conventional DSC, the anodes are typically constructed using thickfilms (˜10 μm) of TiO₂ or, less often, ZnO nanoparticles that aredeposited as a paste and sintered to produce electrical continuity. Thenanoparticle film provides a large internal surface area for theanchoring of sufficient chromophore (usually a ruthenium-based dye) toyield high light absorption in the 400-800 nanometer region, where muchof the solar flux is incident. The chromophore may be in the form of adye monolayer. In operating cells, photons intercepted by the dyemonolayer create excitons that are rapidly split at the nanoparticlesurface, with electrons injected into the nanoparticle film and holesexiting the opposite side of the device via redox species (traditionallythe I⁻/I₃ ⁻ couple) in a liquid or solid-state electrolyte.

The nature of electron transport in oxide nanoparticle films is now wellstudied. Time-resolved photocurrent and photovoltage measurements andmodeling efforts indicate that electron transport in wet, illuminatednanoparticle networks occurs by a trap-limited diffusion process inwhich photogenerated electrons are repeatedly captured and expelled byan exponential distribution of traps as they undertake a random walkthrough the film. Drift transport (which is a mechanism in mostphotovoltaic cells), is prevented in DSCs by ions in the electrolytethat screen all macroscopic electric fields and couple strongly with themoving electrons, effectively rendering them neutral carriers (i.e.,ambipolar diffusion). Under normal solar light levels, an injectedelectron is thought to experience an average of a million trappingevents before either percolating to the collecting electrode orrecombining with an oxidizing species (principally I₃ ⁻ in theelectrolyte). Transit times for electron escape from the film are aslong as a second. Despite the extremely slow nature of suchtrap-mediated transport (characterized by an electron diffusivity,D_(n)≦10⁻⁴ cm² s⁻¹, many orders of magnitude smaller than in TiO₂ or ZnOsingle crystals), electron collection remains favored vis-à-visrecombination due to the even slower multi-electron kinetics of I₃ ⁻reduction on oxide surfaces. Electron diffusion lengths of 10-30 μm havebeen reported for cells operating at light intensities up to 0.1 Sun.This is strong evidence that electron collection is highly efficient forthe 10 μm-thick nanoparticle films normally used in devices. Thesurprising success of the DSC results from this balance between sluggishtransport in the anode and the ultra low recombination rate of electronswith I₃ ⁻. The slow recombination is itself partly due to the excellentelectrostatic screening provided by the liquid electrolyte.

One can gain insight into the factors that limit DSC performance bycomparing the theoretical cell efficiencies with those of currentstate-of-the-art cells. The power conversion efficiency of a solar cellis given as η=(FF|J_(sc)|V_(oc))/P_(in), where FF is the fill factor,J_(sc) is the current density at short circuit, V_(oc) is thephotovoltage at open circuit and P_(in) is the incident light powerdensity. In principle, the maximum J_(sc) of a DSC is determined by howwell the absorption window of its dye sensitizer overlaps the solarspectrum. Record cells achieve currents (and overall efficiencies) thatare between 55-75% of their theoretical maxima at full Sun, depending onthe dye used. Much of the shortfall is due to the poor absorption of lowenergy photons by available dyes. The development of better dyes andlight-trapping schemes has received significant attention in thisregard, so far with little success. A second method of improving theabsorption of red and near-IR light is by thickening the nanoparticlefilm to increase its optical density. This yields diminishing returns asthe film thickness approaches and exceeds the electron diffusion lengththrough the nanoparticle network.

One promising solution to the above impasse is to increase the electrondiffusion length in the anode by replacing the nanoparticle film with anarray of oriented single-crystalline nanowires. Electron transport incrystalline wires is expected to be several orders of magnitude fasterthan percolation in a random polycrystalline network. By using densearrays of long, thin nanowires, one should be able to improve the dyeloading (and so the absorption of red light) while maintaining theexcellent carrier collection characteristics of traditional nanoparticleDSCs. Moreover, the rapid transport provided by a nanowire anode wouldbe particularly favorable for DSC designs that use non-standardelectrolytes, such as polymer gels or solid inorganic phases, in whichrecombination rates are high compared to the liquid electrolyte cell.

To act as an efficient DSC photoanode, a nanowire film preferably has alarge surface area for dye adsorption, comparable to that of itsnanoparticle analogue. In embodiments of the invention, high surfacearea ZnO nanowire arrays were made in aqueous solution using a seededgrowth process that was modified to yield long wires (as describedabove). Briefly, a thin (<15 nanometer) layer of ZnO quantum dots(nanocrystals) was deposited on a surface by dip coating, and wires weregrown from these nuclei via the thermal decomposition of a zinc aminocomplex (as described above). The overall process is a simple, lowtemperature and environmentally benign route to forming dense arrays (upto 40 billion wires per cm²) on arbitrary substrates of any size. Pastreports of solution-grown ZnO nanowires have been limited to lengthsthat are too small for use in efficient DSCs.

ZnO nanowire films are good electrical conductors perpendicular to thesubstrate plane (that is, along the wire axes). Two-point electricalmeasurements of dry arrays on SnO₂:F coated glass give linear I-V tracesthat indicate barrier-free nanowire/substrate contacts. Individualnanowires were extracted from the arrays, fashioned into field-effecttransistors (FETs) using standard electron-beam lithography procedures,and analyzed to extract their resistivity, carrier concentration andmobility. Measured resistivity values ranged from 0.3-2.0Ω cm, fallingon the conductive end of the spectrum for nominally undoped ZnO. Amoderately high electron concentration of 5×10¹⁸ cm⁻³ and mobility of1-5 cm² V⁻¹ s⁻¹ were estimated from transconductance data. Using theEinstein relation, D=k_(B)Tμ/e, the present inventors calculated anelectron diffusivity D_(n)=0.05−0.5 cm² s⁻¹ for single dry nanowires.This value is 500 times larger than the best diffusivity for TiO₂ or ZnOnanoparticle films in operating cells. Faster diffusion in nanowires isa consequence of their excellent crystallinity and complete lack ofgrain boundaries, as confirmed by transmission electron microscopy (notshown).

A DSC cell according to an embodiment of this invention includes anoptimized nanowire array deposited on conductive glass. A monolayer ofdye molecules is formed on the nanowire array. For example, a dye may beadsorbed on the nanowire array using a vapor phase or a liquid phasecoating process. Suitable dyes include ruthenium based dyes including[(CN)(bpy)₂Ru—CN—Ru(dcbpy)₂-NCRu(bpy)₂],[Ru(4,4-bis(carboxy)-bpy)₂(NCS)₂] and [Ru(2,2′,2″-(COOH)₃-terpy)(NCS)3].Other commercially available dyes may be used instead.

The nanowires may comprise any of the materials or characteristicsmentioned above. For example, the nanowires may comprise zinc oxide,titanium oxide, tin oxide, core/shell nanowires (e.g., titanium oxide onzinc oxide), or any other suitable semiconductor material in anysuitable configuration.

After it is formed, the one half of the cell that includes the nanowiresis then sandwiched together with a counter-electrode (e.g., conductiveglass coated with thermalized platinum particles) using a hot-meltpolymer spacer (or other type of spacer). The interior space is thenfilled with an electrolyte solution such as an I⁻/I₃ ⁻ redox couple in anitrile-based solvent. Other types of electrolyte solutions are known tothose of skill in the art and can be used in embodiments of theinvention. The electrolyte may alternatively be a polymer gel. Inaddition, the substrates could alternatively be conductive plastic,instead of coated, conductive glass.

FIG. 9A shows a conventional nanoparticle DSC 30. As illustrated, anumber of semiconductor particles are present between the twoelectrodes.

FIG. 9B shows a DSC 20 according to an embodiment of the invention. TheDSC 20 includes a first conductive substrate 10 including an (fluorinedoped F:SnO2) SnO₂ electrode, and a second conductive substrate 14including an (fluorine doped F:SnO2) SnO₂ electrode with a Pt mirror. Inbetween the first and second conductive substrates 10, 14, is an arrayof nanowires 12 with a dye coating. A liquid electrolyte 18 is betweenthe nanowires in the array and is also between the first and secondconductive substrates 10, 14.

FIG. 10 shows a graph of current density (mA cm⁻²) vs. bias voltage (V)for a DSC. A roughness factor=200. It shows characteristics of a DSCsolar cell showing 1.5% efficiency under 100 mW/cm² of simulatedsunlight.

FIG. 11 is a graph of cell efficiency vs. roughness factor. It showsthat a larger surface area will improve efficiency. “Roughness factor”is unit-less and is the surface area of a sample per geometric area ofthe substrate.

B. Organic-Inorganic Hybrid Photovoltaic Devices

Embodiments of the invention can also include organic-inorganic hybridcells. Currently, to separate the donor and acceptor materials in suchcells (or devices), a spin cast method is used. However, this creates adisordered film morphology, which causes poor transport properties inthe cells. The use of nanowires provides an ordered bulk interface witha direct pathway to the electrode, thus reducing poor transportproperties and improving the performance of the cell.

The organic-inorganic hybrid photovoltaic cells according to embodimentsof the invention are particularly promising for efficient excitonicphotoconversion. In these devices, an array of thin (about 20-30nanometers in diameter), short (about 100-300 nanometers tall) nanowiresmay be intimately wetted with a conducting or semiconducting polymer(e.g., P3HT) that penetrates into the spaces (e.g., 10-50 nanometerspaces) between the wires. The polymer acts as both a light-harvestingand a hole conducting material. Electrons are captured by the ZnO wiresand are transported to the anode. The device includes theorganic-inorganic composite sandwiched between two electrodes.

The dense nanowire arrays according to embodiments of the invention havehigh surface area and pore spaces large enough to form a high-qualityjunction with the polymer layer. The nanowires contact the electrodedirectly and provide an excellent current pathway.

FIG. 12 shows a schematic view of an array of nanowires from a top view.2L_(D) represents the spacing between adjacent nanowires and may be 20nanometers in some embodiments. L_(D) is the exciton diffusion length.In order for the device to efficiently split the exciton at apolymer-semiconductor interface by not having the electron-hole pairrecombine, the spacing between the semiconductor wires should be ideallyless than 2L_(D).

FIG. 13 shows a schematic illustration of an organic-inorganic hybridcell according to an embodiment of the invention. The cell includes afirst conductive layer 62, which comprises a transparent or translucentmaterial to allow for the passage of light, and a second conductivelayer 64. The second conductive layer 64 may comprises a reflective,conductive material such as gold or silver (or alloys thereof).Alternatively, the second conductive layer 64 may comprise a transparentor translucent material. A reflective film may on the transparent ortranslucent second conductive layer 64. By providing a second conductivelayer 64 that has some reflective characteristics, light that passesthrough the first conductive layer 62 and a wire composite 66 betweenthe first conductive layer 62 and the second conductive layer 64 isreflected back to the wire composite 66, thereby maximizing thetransmission of light to the wire composite 66. Illustratively, thefirst conductive layer 62 may comprise ITO, while the second conductivelayer 64 may comprise gold or silver. A transparent or translucentsubstrate 68 may support the second conductive layer 64. The substrate68 may comprise a rigid material such as glass. Electrical connections70(a), 70(b) may be provided to the first and second conductive layers62, 64.

The wire composite 66 is between the first conductive layer 62 and thesecond conductive layer 64. The wire composite 66 comprises any of thenanowire arrays described herein in combination with an electricallyconductive or semiconducting polymer.

The phrase “semiconducting polymer” includes all conjugated polymershaving a pi-electron system. Non-limiting examples of semiconductingpolymers include trans-polyacetylene, polypyrrole, polythiophene,polyaniline, poly (p-phenylene and poly(p-phenylene-vinylene),polyfluorenes, polyaromatic amines, poly(thienylene-vinylene)s andsoluble derivatives of the above. An example is(poly(2-methoxy,5-(2′-ethylhexyloxy)p-phenylenevinylene) (MEH-PPV) andpoly(3-alkylthiophene) (e.g., poly(3-hexylthiophene) or P3HT). Someembodiments of the invention can also use conjugated polymers that areeither solution processable or melt processable, because of bulk pendantgroups attached to the main conjugated chain or by its inclusion of theconjugated polymer into a copolymer structure of which one or morecomponents are non-conjugated. Non-limiting examples includepoly(,4′-diphenylenediphenylvinylene),poly(1,4-phenylene-1-phenylvinylene andpoly(1,4-phenylenediphenylvinylene, poly(3-alkylpyrrole) andpoly(2,5-dialkoxy-p-phenylenevinylene). It is understood that the term“semiconducting conjugated polymer” could include a mixture or blend ofpolymers, one of which is to be a semiconducting conjugated polymer.

The cell shown in FIG. 13 can be used in any suitable manner. Forexample, in some embodiments, light passes through the substrate 68 andthe second conductive layer 64 and irradiates the wire composite 66comprising the nanowire array. This, in turn, induces current flowthrough the electrical connections 70(a), 70(b).

The cell shown in FIG. 13 can be made in any suitable manner. Forexample, in one embodiment, one may coat the substrate 68 with the firstconductive layer 62 using a conventional coating process such as vapordeposition, electroplating, electroless plating, etc. After obtainingthe coated conductive substrate, an array of nanowires can be grown onthe first conductive layer 62 in the manner described herein. Then, aconductive or semiconductive polymer may be deposited on the array ofnanowires, and can fill the spaces between the nanowires. The polymermay be deposited on the array of nanowires using any suitable processincluding roller blade coating, surface initiated polymerization, dipcoating, spin coating, vapor deposition, etc. After forming the wirecomposite 66, the second conductive layer 64 can be formed on the wirecomposite 66. The second conductive layer 64 can be formed using thesame or different process as the first conductive layer 62. Then, theelectrical connections 70(a), 70(b) can be attached to the first andsecond conductive layers 62, 64.

The structure shown in FIG. 13 can be produced by other processes. Inanother example, the wire composite 66 can be formed first, and then thesubstrate 68 and the first and second conductive layers 62, 64 can belaminated together. Once they are laminated together, the electricalconnections can be attached to the first and second conductive layers62, 64.

FIG. 14 shows SEM photographs of actual wire composites includingnanowire arrays and a polymeric material.

FIG. 15 is an x-ray diffraction plot associated with anorganic-inorganic hybrid cell including a wire composite including azinc oxide nanowire array and P3HT (poly(3-hexylthiophene)), an ITOconductive layer, and glass. As shown in FIG. 15, P3HT retains itscrystalline domains, therefore maintaining high mobility.

FIG. 16 shows electrical characteristics for a single ZnO nanowire. I-Vcurves at various gate biases for a nanowire with a diameter of 75 nm,showing n-type behaviour and a resistivity of 0.65Ω cm. The left insetshows the corresponding I_(SD)-V_(G) curve at V_(SD)=100 mV. The ON-OFFratio is 10⁵ at 50 volts. The right inset shows an SEM image of the ZnONW-FET.

C. Solid State Sensitized Solar Cells

Solid state sensitized solar cells are also referred to asdye-sensitized heterojunctions (DSHs). These have a structure similar tothe DSCs described above, but in a DSH, a light absorbing dye is placedat an n-p heterojunction. DSHs without nanowire arrays are described inRegan, et al., “A solid-state dye-sensitized solar cell fabricated withpressure-treated P25-TiO₂ and CuSCN: analysis of pore filling and IVcharacteristics”, Chemistry of Materials 14 (12): 5023-5029 (January2003), which is herein incorporated by reference in its entirety.

FIG. 17 shows a solid state sensitized solar cell with a nanowireelectrode. It includes a substrate 212 and a first conductive layer 204on the substrate. A second conductive layer 206 in the form of a metalcontact is on a solid-state semiconductor material such as a p-typesolid-state semiconductor 202. Any suitable p-type semiconductor may beused. For example, a p-type semiconductor such as CuI or CuSCN may beused. It may alternatively be an amorphous organic hole transmittingmaterial. The solid state semiconductor material 202, an electronblocking layer 208 and a nanowire array 210 are between the firstconductive layer 204 and the second conductive layer 206. The nanowirearray 210 may comprise ZnO or any of the materials mentioned above, andmay be coated with a dye (not shown). An electron blocking material suchas AlGaN can be used for the electron blocking layer 208. The electronblocking layer 208 improves the efficiency of the device. Thesemiconductor material 202 fills the spaces between the nanowires in thenanowire array 210.

The solid state sensitized cell shown in FIG. 17 has a constructionsimilar to the previously described cells, except that an electronblocking layer is present. The same processes and materials that aredescribed above with respect to the DSC and the hybrid cell can be usedto construct the cell shown in FIG. 17. For example, the semiconductormaterial 202 may be deposited on the nanowire array 210 usingconventional coating processes. The The blocking layer 208, if present,may be formed on the substrate 212 by any suitable known coating processsuch as a vapor phase coating process (e.g., sputtering and CVD).

D. Light Emitting diodes

Embodiments of the invention can also be used to produced LEDs or lightemitting diodes.

FIG. 18 shows a light emitting diode according to an embodiment of theinvention. It includes a substrate 312 and first conductive layer 304 onthe substrate. A second conductive layer 306 in the form of a metalcontact is on a solid state semiconductor material such as a p-typesolid state semiconductor 302. Any suitable p-type semiconductor may beused. For example, a p-type semiconductor such as CuI or CuSCN may beused. The solid state semiconductor material 302, an electron blockinglayer 308 and a nanowire array 310 comprising an n-type semiconductorsuch as ZnO are between the first conductive layer 304 and the secondconductive layer 306. An electron blocking material such as AlGaN can beused. The semiconductor material 302 fills the spaces between thenanowires in the nanowire array 310.

As is known in the art, when an electrical bias is provided to a p-njunction, the recombination of electron-hole pairs injected into adepletion region causes the emission of electromagnetic radiation.Accordingly, the device shown in FIG. 18 may be coupled to a powersource (not shown) to provide the electrical bias to the p-n junctionformed at the nanowire 310/semiconductor material 302 interface.

The LED has a construction similar to the previously described cells,except that a dye need not be present. The same processes and materialsthat are described above with respect to the other cells and devices maybe used to construct the device shown in FIG. 18.

III. Branched Networks Including Nanowires

Many of the above-described embodiments include an array of nanowireswithout branches. Other embodiments of the invention are directed tonanowire arrays including branches. These may be used to form athree-dimensional network on a substrate.

FIG. 19 shows an exemplary flowchart illustrating a method according toan embodiment of the invention. As shown there, a substrate is provided(step 400). Nanocrystals are then deposited on the substrate, preferablythrough a self assembly process (step 402). After depositing thenanocrystals on the substrate, a solution then contacts the depositednanocrystals. After the solution contacts the deposited nanocrystals, anarray of nanowires is formed (step 404). Process steps 400, 402, and 404and other suitable process steps are already described above.

After forming an array of nanowires on a substrate, the array ofnanowires and substrate can be removed from the solution and dried.Additional nanocrystals are deposited on the nanowire array (step 406).After depositing the nanocrystals on the nanowire array, they can becontacted with the solution (as previously described) and branches canform from the deposited nanocrystals while in the solution (step 408).The process that is used to form the nanobranches on the nanowires maybe the same or different process that is used to form the nanowiresthemselves. Nanowire formation processes are described in detail aboveand below, and the details of those process are incorporated in thisdiscussion of forming nanobranches. This nanobranch formation processcan be repeated as often as desired to form branches on the previouslyformed branches to form a three-dimensional network of branchednanowires (step 410). It is understood that any of the process stepsdescribed above with respect to the formation of the nanowire arrays canbe used to form the nanobranches.

FIG. 20 shows a schematic illustration of another three-dimensionalnetwork 80 of branched nanowires. First, the nanowires 92 are grown onthe illustrated substrate. Then, a first set of nanobranches 94 isformed on the nanowires. Then, a second set of nanobranches 96 can beformed on the first set of nanobranches and/or the nanowires 92.Additional sets of nanobranches can be formed after that.

The branched nanowire arrays according to embodiments of the inventionhave a high surface area. This is particularly desirable. For example, aphotovoltaic cell with a nanowire array with a higher surface areagenerally has a better energy conversion efficiency than a photovoltaiccell with a nanowire array with a lower surface area.

Some additional examples are provided below.

IV. Examples

A. Nanowire Characterization

Nanowire arrays were formed according to the solution based processesmentioned above (without polyethylenimine). U.S. Provisional PatentApplication No. 60/480,256, filed on Jun. 20, 2003, which is hereinincorporated by reference, provides further details on these and otherexamples.

The ultraviolet and visible photoluminescence (PL) of as-grown nanowirearrays was measured in the temperature range 4.5≦T≦300 K using alow-power, unfocused 325 nm line of a He—Cd laser as the excitationsource. Room temperature spectra of as-grown samples showed a weak bandedge emission at 378 nm (3.29 eV), due to free exciton annihilation, anda very strong and broad yellow-orange emission that is fit well by twoGaussians, with a major peak centered at 605 nm (2.05 eV) and a shoulderat 730 nm (1.70 eV). The three peaks grow more intense with decreasingtemperature as a result of the freeze-out of phonons and quenching ofnonradiative recombination processes. A 90 meV blueshift of theband-edge emission over this temperature range was caused by the thermalcontraction of the lattice and changing electron-phonon interactions(Zhang et al., J. Lumin., 99:149 (2002)). The temperature dependence ofthe orange (2.05 eV) photoluminescence intensity can be expressed by asimple thermal activation model of the form (Jiang et al., J. Appl.Phys., 64:1371 (1988)),I=I _(O)/(1+A*exp(−E _(A) /k _(B) T)).   (1)By fitting the experimental data, an activation energy E_(A)=71 meV wasobtained for the nonradiative mechanisms responsible for quenching theorange luminescence. This value is almost three times greater than theenergy reported in a previous study of single crystal and powder samples(Lauer, J. Phys. Chem. Solids, 34:249 (1973)).

It is known that pure ZnO can show green and/or orange visibleluminescence depending on the growth temperature and availability ofoxygen during sample preparation (Zelikin et al., Optika iSpektroskopiya, 11:397 (1961); Bhushan et al., Indian J. Pure & Appl.Phys., 19:694 (1981)). The green emission is due to the recombination ofelectrons with holes trapped in singly-ionized oxygen vacancies (V_(o)⁺) and is commonly seen in ZnO materials synthesized under oxygendeficient conditions, including the gas-phase nanowires produced (vanDijken et al., J. Lumin., 90:123 (2000)). Orange photoluminescence hasbeen seen in ZnO grown electrochemically (Zheng et al., Chem. Phys.Lett., 363:123 (2002)), hydrothermally (Sekiguchi et al., J. CrystalGrowth, 214/215:72 (2000)), and via pulsed laser deposition (Wu et al.,Appl. Phys. Lett., 78:2285 (2001)) and spray pyrolysis (Studenikin etal., J. Appl. Phys., 84:2287 (1998)). The strong orange PL and completeabsence of green emission from the aqueous-grown nanowire arrays isconsistent with the above assignments. Regardless of the exact origin ofthe orange emission, the large ratio of orange PL intensity to band-edgePL intensity indicates that the as-grown nanowires are rich in defects,which is typical for crystals grown in solution.

Photoluminescence and lasing measurements were combined with a series ofannealing treatments in order to investigate the nature of the orangeemission. Three samples were cut from both a 1.5-hour nanowire array anda 3-hour nanowire array on silicon. The samples were then annealed inone of three environments, either 400° C. in 10% H₂/90% Ar for 15minutes, 500° C. in 10% H₂/90% Ar for 15 minutes, or 800° C. in a 5×10⁻⁶Torr vacuum for 2 hours. One sample from each wafer was left untreatedas a control. Post-anneal SEM imaging of each sample confirmed that thenanowires survived the annealing processes and were visibly undamaged,except for a slight surface etching seen in the 500° C. H₂ treatments.Room temperature photoluminescence spectra of the 1.5-hour samplesshowed a progressive quenching of the orange emission with asimultaneous increase of the band-edge PL intensity. The markedweakening of the orange emission after vacuum annealing is consistentwith the involvement of oxygen interstitials in the luminescence. The500° C. H₂ treatment caused a nearly complete quenching of the orange PLand resulted in a spectrum dominated by band-edge emission. The green PLfeature, which should develop with sufficiently reducing treatments,(Studenikin et al., J. Appl. Phys., 84:2287 (1998)) was not observedeven in a sample that was exposed to hydrogen at 600° C. and appearedheavily etched by SEM.

The lasing behavior of the eight array samples was investigated byfar-field photoluminescence imaging using an experimental setupdescribed previously (Johnson et al., Nature Materials, 1:101 (2002)).Neither of the as-grown samples showed lasing at sub-ablation pumpingintensities. Lasing of array nanowires on the annealed 1.5-hour sampleswas observed in only a small fraction of the wires, likely those at theupper limit of the diameter distribution having a sufficient cavityfinesse to support a single lasing mode. The annealed 3-hour samples(average diameter d=125 nm, length 2 μM) showed lasing in roughly tentimes the number of wires as the 1.5-hour arrays (average diameter 60nm, length 1.5 μm), which is reasonable since optical gain in a nanowirescales exponentially with the cavity length and decreases with d<λ, dueto diffraction effects. A typical PL spectrum of a 3-hour solution arrayabove threshold showed several relatively sharp lasing peakssuperimposed on a broad PL background. However, the percentage of lasingwires in the 3-hour arrays remained very low, probably because theirshort lengths lead to high gain thresholds. A comparison of the averagelasing thresholds for the active wires on the three annealedsolution-grown arrays and a gas-phase array showed that the thresholdtends to decrease as the ratio of ultraviolet PL to visible PLincreases, with the gas-phase array lasing at one-fifth the threshold ofthe most reduced 3-hour sample. Annealing as-grown arrays in reducingatmospheres quenches the intense orange emission and lowers the lasingthreshold of the larger nanowires to values similar to gas-phasesamples.

B. Nanowires Formed Using Polyethylenimine

Synthesis of ZnO Nanowire Photoanodes: ZnO nanowire arrays weresynthesized on ITO-coated glass substrates (10Ω/sq, Thin Film Devices,Inc.) and/or fluorine doped SnO₂ (7-8Ω/sq, Hartford Glass) using amodified version of a published approach. After sonication cleaning inacetone/ethanol and 1.0 M HCl, the substrates were manually dip-coatedin a solution of ZnO quantum dots in ethanol, rinsed with ethanol anddried in a stream of gas. This procedure reliably formed a dense layerof nanoparticle seeds on the surface. Nanowire arrays were then grown inaqueous solutions containing 20-60 mM zinc nitrate hexahydrate, 25-50 mMhexamethylenetetramine and 0.5-5 mM polyethylenimine (branched, lowmolecular weight, Aldrich) at 90-95° C. for 2-5 hours. Since nanowiregrowth ceased after this period, substrates were repeatedly introducedto fresh solution baths in order to attain long wires and high surfaceareas (total reaction times of 15-25 hrs). The array electrodes werethen rinsed with deionized water and baked in air at 400° C. for 1 hourto remove any residual polymer.

Using polyethylenimine (PEI) in the solution, the aspect ratios of thenanowires increased to above 120. The dramatic effect of this moleculein solution can be seen in FIGS. 7, 8A, and 8B. FIG. 7 correlatesnanowire length and diameter at different growth times with and withoutPEI addition.

Electrical measurements: For the single wire studies, 8-10 μm long ZnOnanowires were dispersed from solution onto oxidized silicon substrates(300 nm SiO_(x)) and fired in air at 400° C. for 15 minutes.Electron-beam lithography was used to pattern and deposit 100 nm thickTi contacts from the wires to prefabricated contact pads. Many devicesshowed ohmic I-V plots without further heat treatment. Measurements wereperformed with a global back gate using a semiconductor parameteranalyser (HP4145B, HP).

Samples for array transport studies were made by encapsulating firedarrays (grown on ITO) in a matrix of spin-cast poly(methyl methacrylate)(PMMA), exposing the wire tips by UV development of the top portion ofthe PMMA film, and then depositing metal contacts by thermalevaporation. The insulating PMMA matrix prevented potential shortcircuits due to pinholes in the nanowire film and provided mechanicalstability for the measurement.

Mid-infrared transient absorption measurements: Mid-IR transientabsorption measurements were performed with a home-built Ti:sapphireoscillator (30 fs, 88 MHz) and commercial regenerative amplifier(Spectra-Physics, Spitfire) that operates at 810 nm and 1 kHz repetitionrate. Approximately 800 μJ of the beam was used to pump an opticalparametric amplifier (OPA) (TOPAS, Quantronix), while 80 μJ was retainedand frequency-doubled in BBO for use as the 405 nm pump beam. This beamwas temporally delayed by a motorized stage and directed to the sample.The signal and idler beams from the OPA were combined in a AgGaS₂crystal to create tunable mid-IR pulses (1000-3500 cm⁻¹). The residual810 nm beam and the residual signal and idler beams were re-combined ina BBO crystal to create SFG at 510 nm and 575 nm. The 510 nm beam wasdirected to a separate delay stage and then to the sample. The pumpbeams were focused to a spot size of approximately 200-300 μm, withtypical pump energies of 0.5-2 μJ. The pump beams were mechanicallychopped at 500 Hz (synchronous with the laser), and separate boxcarintegrators were triggered by the rejected and passed beams, allowingfor independent detection channels of probe with pump (“sample”) andwithout pump (“reference”). The sample signal was subtracted from thereference signal, and the result was divided by the reference to givethe differential transmittance, which was converted to effectiveabsorbance. The probe beam, which was typically centered at 2150 cm⁻¹with a 250 cm⁻¹ bandwidth, was focused with a CaF₂ lens to a size ofapproximately 100-200 μm. The probe beam was collected aftertransmission through the sample and directed through bandpass filtersbefore being focused onto a single-element MCT detector (IR Associates).An instrument response of 250-300 fs was determined by measuring thesub-50 fs rise of free-electron absorption in a thin Si wafer after blueor green pump.

Each transient plot (not shown) was an average of points taken on bothforward and reverse scans (checked for reproducibility). Each pointconsists of approximately 500 averaged laser shots. Samples weretranslated after each scan in order to minimize probing dyephotoproducts. However, samples were not moved during the scan becausesmall inhomogeneities caused changes in the amplitude of the transientsignal, obscuring the true kinetics.

Fabrication of Solar Cells: As-made nanowire films were first fired at400° C. in air for 30 minutes to remove surface adsorbates and thendye-coated in a 0.5 mmol 1⁻¹ solution ofcis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)(N3 dye) in dry ethanol for three hours at 25° C. The cells wereconstructed by sandwiching nanowire anodes together with thermallyplatinized conducting glass counter electrodes separated by 30-50 μmthick hot-melt spacers (Bynel, Dupont) and sealed by flash heating. Theinternal space of a cell was filled by injecting a liquid electrolyte(0.5 M LiI, 50 mM I₂, 0.5 M 4-tertbutylpyridine in3-methoxypropionitrile).

The terms and expressions which have been employed herein are used asterms of description and not of limitation, and there is no intention inthe use of such terms and expressions of excluding equivalents of thefeatures shown and described, or portions thereof, it being recognizedthat various modifications are possible within the scope of theinvention claimed. Moreover, any one or more features of any embodimentof the invention may be combined with any one or more other features ofany other embodiment of the invention, without departing from the scopeof the invention. For example, any features of any of the nanowirearrays described herein can be combined with any features of any of theoptoelectronic devices described herein without departing from the scopeof the invention.

All patents, patent applications, and publications mentioned above areherein incorporated by reference in their entirety for all purposes.None of the patents, patent applications, and publications mentionedabove is admitted to be prior art.

1. An electronic device, comprising: a first conductive layer; an arrayof nanowires, the nanowires having first ends and second ends, the firstends adjacent the first conductive layer, the nanowires extendingapproximately perpendicular to the first conductive layer, wherein thenanowire array has a density of approximately 10¹⁰ nanowires per cm²; asecond conductive layer approximately parallel to the first conductivelayer and adjacent the second ends of the nanowires; and an electronicmaterial filling spaces between and around the nanowires.
 2. The deviceof claim 1 wherein the first conductive layer is either transparent ortranslucent.
 3. The device of claim 1 wherein the first conductive layercomprises conductive glass or plastic.
 4. The device of claim 1 whereinthe first conductive layer comprises fluorine doped SnO₂ or ITO.
 5. Thedevice of claim 1 wherein the nanowires comprise ZnO, TiO₂, SnO₂, orcombinations thereof.
 6. The device of claim 1 wherein the nanowireshave a shell/core structure comprising TiO₂/ZnO.
 7. The device of claim1 wherein the nanowires have diameters of approximately 20-30 nm andlengths of approximately 1-3 μm.
 8. The device of claim 1 wherein thenanowires have diameters of approximately 40-80 nm and lengths ofapproximately 1.5-2 μm.
 9. The device of claim 1 wherein the nanowireshave diameters of approximately 200-300 nm and lengths of approximately3 μm.
 10. The device of claim 1 wherein the nanowires in the nanowirearray have a nanowire-to-nanowire spacing of approximately 10-50 nm. 11.The device of claim 1 wherein the nanowires in the nanowire array have ananowire-to-nanowire spacing of approximately 20 nm.
 12. The device ofclaim 1 wherein the electronic material comprises an electrolytesolution.
 13. The device of claim 12 wherein the electrolyte solution isa polymer gel.
 14. The device of claim 1 wherein the electronic materialcomprises a semiconducting polymer.
 15. The device of claim 14 whereinthe semiconducting polymer is selected from the group consisting oftrans-polyacetylene, polypyrrole, polythiophene, polyaniline, poly(p-phenylene and poly(p-phenylene-vinylene), polyfluorenes, polyaromaticamines, poly(thienylene-vinylene)s, (poly(2-methoxy,5-(2′-ethylhexyloxy)p-phenylenevinylene), poly(3-alkylthiophene),poly(,4′-diphenylenediphenylvinylene),poly(1,4-phenylene-1-phenylvinylene andpoly(1,4-phenylenediphenylvinylene, poly(3-alkylpyrrole),poly(2,5-dialkoxy-p-phenylenevinylene), and mixtures or blends thereof.16. The device of claim 1 wherein the electronic material comprises ap-type semiconductor.
 17. The device of claim 16 wherein the p-typesemiconductor is CuI or CuSCN.
 18. The device of claim 1, furthercomprising an electron blocking layer around the first ends of thenanowires and adjacent the first conducting layer.
 19. The device ofclaim 18 wherein the electron blocking layer comprises AlGaN.
 20. Thedevice of claim 1, further comprising a monolayer of dye molecules onthe nanowire array.
 21. The device of claim 20 wherein the dye moleculesare selected from the group consisting of[(CN)(bpy)₂Ru—CN—Ru(dcbpy)₂-NCRu(bpy)₂],[Ru(4,4-bis(carboxy)-bpy)₂(NCS)2], and [Ru(2,2′,2″-(COOH)₃-terpy)(NCS)3].
 22. The device of claim 1, further comprising a reflectivecoating on the second conductive layer.
 23. An electronic device,comprising: a first conductive layer; an array of nanowires havingn-type behavior, the nanowires having first ends and second ends, thefirst ends adjacent the first conductive layer, the nanowires extendingapproximately perpendicular to the first conductive layer; a secondconductive layer approximately parallel to the first conductive layerand adjacent the second ends of the nanowires; and a p-typesemiconductor material filling spaces between and around the nanowires.24. The device of claim 23 wherein the first Conductive layer is ITO.25. The device of claim 23 wherein the nanowires comprise ZnO and thesemiconductor material comprises CuI or CuSCN.
 26. The device of claim23 wherein the nanowire array has a density of approximately 10¹⁰nanowires per cm².
 27. The device of claim 23 wherein the secondconductive layer is reflective.
 28. The device of claim 23, furthercomprising an electron blocking layer around the first ends of thenanowires and adjacent the first conducting layer.
 29. The device ofclaim 28 wherein the electron blocking layer comprises AlGaN.